The present invention relates to a method for making a sound circumferential weld at the overlapped joining region of a metal tube-receiving member with an inserted tube. In a particular application, the present invention relates to a practical method for forming such a hydrostatic pressure-tight connection in an assembled tubular structure intended to undergo subsequent hydroforming.
There are a variety of manufacturing or assembly applications in which the end of one metal tube is inserted into the complementary cylindrical recess of another member for permanent attachment to it. In many applications the receiving member is a second tube with an end sized to receive the end of the first tube. In other applications, for example, the second member is a structural member, such as a nodal piece of a tubular frame structure with an opening adapted to be inserted into, or to receive, the end of one or more such tubes.
Frame structures comprising an assembly of tubes and tube-end attachment members have been used in bicycle frames and small airplane bodies and the like. Such structures are also being considered for car bodies as disclosed in U.S. Pat. No. 5,720,092, assigned to the assignee of this invention. In the automotive application, it may be the practice to assemble a group of round metal tubes with each other and tube-end receiving node pieces into an initial frame assembly. The initial assembly is placed between matched dies, filled with water and subjected to sufficient hydrostatic pressure to expand or bend or otherwise reshape the initial tubular assembly into an integral frame member of complex three-dimensional shape. In this hydroforming procedure, it is necessary to have high pressure water-tight connections between tubes or between tubes and mating structural members. Heretofore there has not been a practical and reliable method of obtaining such connections.
This invention provides a method of providing a sound circumferential weld between the end of a tube and a second member adapted to receive either the inside or outside dimension of the tube end in suitably tight engagement. The pieces to be welded may be two tubes with inter-fitting ends or a tube and a tube-end receiving member. The tube-end receiving member may be sized to receive the outside diameter of the tube, or the tube receiving member may itself have a tube portion that fits within the inside diameter of the tube. While the following description is largely expressed in terms of two tubes for simplicity, the subject method is applicable to both of the above-described applications. As stated, a principal purpose and application of the invention is in assembling tubular frame structures for subsequent hydroforming.
In many, but not necessarily all, applications, the starting workpieces will be round tubes with the outside diameter of the end of the first tube closely fitting the inside diameter of the end portion of the second tube. Of course, the end portions of such tubes can be of any suitable cross section so long as they are complementary, or geometrically similar, for insertion of one into the other. For example, the tubes could be square, rectangular, oval, or the like. Again, for simplicity, in the following description the tubes will often be referred to as round, but the applicability of this process is to tubes of complementary end portions in general.
According to a first embodiment of the invention, a first tubular member is provided having an end with a predetermined, desired outer diameter (sometimes OD herein), or other defining dimension if the tube is not round, and a second tubular member is provided having an internal diameter (sometimes ID herein) slightly larger (for example, about two millimeters larger) than the OD of the first tube. A continuous, circumferential, inwardly directed, shallow protrusion is formed in the end region of the second tube. Preferably, the inside dimension of the annular protrusion is such that it will tightly receive the OD surface of the first tube. Thus, where there is about two millimeters difference in the diameters of the complementary tubes, the protrusion extends radially about one millimeter in from the ID of the outer tube. The end of the first tube is inserted into the end of the second tube through the surrounding protrusion. Initially, the protrusion serves to locate and grip the inserted tube. Then a suitable welding current is passed through the overlapping portions of the assembled tubes and through the projection to form a full circumferential bond between the tubes.
In a preferred embodiment, radial pressure is concurrently applied to the outer tube to press its protrusion against the inner tube and promote a full circumferential welded bond between the facing surfaces of the mated tubes. Such pressure may, for example, be applied mechanically with a tool(s) of suitable circumferential shape, or by non-contact means as with a circumferential electromagnetic forming tool.
According to a second embodiment of the present invention, the protrusion is formed on the OD surface of the inner tube of the assembly. The annular protrusion extends outwardly for full circumferential engagement with the inner ID surface of the outer tube. After the end of the outer tube has been pushed over the protrusion of the inner tube, a suitable welding current is passed through the joined pieces and projection to accomplish a full circumferential weld at the region of the projection.
Complementary projections could be formed on both tubes so that, for example, when the tubes are inserted for welding, the projections lie close and parallel to ultimately provide a wider welded connection between the tubes. As stated, a principle application for the subject method is to form a tubular assembly for subsequent hydroforming.